Are there other fragments aside from the Moon that resulted from the collision of Earth and "Theia" (as it's called in recent main-stream press stories)? And if not, why not? And what happened to "Theia" (like were did it go?)

Has there been any consideration or hypothesizing that Earth may have had a larger orbit (say between Mars/Jupiter) until a large body (Theia) bashed into it, sending Earth closer to the sun along with its new satellite, while also creating the asteroid-belt? Could something like Ceres be the missing "Theia"?

I know it sounds a bit far-fetched, but then so does the whole concept of the Moon being born from a planet collision with Earth (with very little evidence to back it up, at least till recently).

1 Answer
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Let's start answering your question in reverse. Ceres could not be "the missing Theia" (I know you don't say that; I'm addressing a side issue) because of its shape. If an object hit the Earth at an angle (as is currently thought), it would be pretty deformed, if it managed to stay together. If it hit the Earth head on . . . Well, it would almost certainly not survive. So if it did survive the impact, it would be in pretty bad shape. Where would such an object go? Let's investigate the idea that it went into the asteroid belt. That begs a question: Where is it today? Ceres is the largest object in the belt (and we've already ruled it out). If the object was smaller - small enough that it blended in - it would have been captured by the Earth's gravity. So its survival provides a paradox - it would have to be big to get out of Earth's orbit, but nothing that big is known. So chances are, Theia didn't survive the impact. The gravitational pull of the Earth also implies that any fragments surviving the collision would have to be moving pretty fast to escape - so fast that it seems unlikely that they stayed in a stable orbit.

Now for your belt-formation theory. It is, admittedly a very cool idea. The mass of the asteroid belt is about 4% that of the Moon - so it could be the result of the planetary crash. And some theories say that the belt is, in fact, the remains of a planet. There is, of course, one problem - the different composition of the asteroids. There are three types, differentiated by their composition - C, S, and M (their properties can be found online). So it is very unlikely that they came from the same planet.

Finally, your idea about the Earth having an orbit farther out. This one seems to have a good chance of being right. The early protoplanets in the solar system were very - very - unstable in their orbits because of the frequency of collisions. So the Earth could have been in a different orbit, and then migrated inwards or outwards.

I'm saying that if a body the size of Ceres were deformed into some non-spherical shape, it would re-form itself into a sphere over time. Any significant irregularities would not hold up against gravity. We have at least one Hubble photo showing it to be close to spherical; this is not a coincidence. I don't suggest that it's plausible that Ceres is a remnant of the Theia/Earth collision, but its shape doesn't argue against it.
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Keith ThompsonAug 11 '14 at 20:54